To make the filling: Place the sushi rice and water in a small saucepan. Bring to a boil, lower the heat and simmer, covered, for 8 minutes. Remove from the heat and let rest, covered, for 10 minutes. Season with vinegar, salt, and pepper. Mix well and cool.

To make the sauce: Bring all the sauce ingredients to a boil in a small enamel-lined saucepan. Season to taste with salt and pepper.

To make the rolls: Lightly salt and pepper each chicken breast on both sides and place it on a piece of cling wrap. Remove the stems from the spinach leaves and flatten the leaves so they will roll easier. Line each breast with 3 spinach leaves and one-fourth of the filling.

Starting with the narrowest end, roll the breast up (not too tight!) until it looks like a log. (I use the cling wrap to facilitate the rolling.) When the breast is rolled and completely enclosed in the cling wrap, twist the sides and close them with a metal tie. Refrigerate if not using right away.

To cook the rolls: Bring the chicken rolls back to room temperature, if necessary. Place them in the basket of a bamboo steamer. Set the basket over a large pot or wok, whose bottom third has been filled with water. Bring the water to a rolling boil. Cover and steam over high heat for 9 to 10 minutes, turning the rolls once. Cook until the chicken has turned pale pink inside. Turn off the heat and let rest, covered, for 1 minute.

To serve: Remove one of the ties, and, holding the other end, slip each roll out onto a plate, Pour off the accumulated juices. Cut each roll on the diagonal into 3 pieces. Place the pieces on a dinner plate or serving dish. Reheat the sauce and spoon the hot sauce over the pieces.